The Chazy Reef Formation is known worldwide among paleontologists as the oldest fossil reef in the world, extending from lower Quebec to northern Alabama. It was formed during the Ordovician Period, some 450-480 million years ago in a shallow tropical sea (this part of North America was below the Equator at the time). Several ages of reef construction are visible in the various layers exposed by quarrying operations here over the years.
Clearly visible in the quarry walls are fossil remains of stromatoporoids (an extinct ancestor of the sponge), one of the builders the Chazy Reef. The quarry is designated a National Natural Landmark; the Fisk Quarry Preserve is protected by the Isle La Motte Preservation Trust.
Further description can be found at this Vermont Geological Survey site.
Before quarrying ceased here, the distinctive grey-black limestone was shipped all over the country; it was used to build the Brooklyn Bridge, Radio City Music Hall, and the National Gallery of Art.